Heat shield blocks for a manned space capsule are made to withstand the harsh environments of reentry. The materials that best serve this purpose tend to be very difficult to interrogate using nondestructive testing techniques. As NASA was in the process of building the Orion space capsule, the team was able to inspect a number of discontinuities in the heat shield but was faced with great difficulty in inspecting their bond quality to the composite substrate. While radiographic computed tomography and terahertz were successful in detecting three-dimensional defects such as voids and porosity, they were unable to detect two-dimensional defects, such as kissing unbonds. Consequently, a customized ultrasonic technique was developed by carefully selecting every aspect of the inspection, including couplant and transducer. Several signal processing techniques were then performed to counteract the effects of inhomogeneity and irregular scatter in the acoustic signal. The technique takes into consideration the shape of the raw A-scan, phase of the returned echo, and confidence in the received signal. Additionally, the technique may perform synthetic aperture focusing to enhance the quality of the scans and provide for a relatively reliable technique for detecting kissing unbonds and other discontinuities in the heat shield material.
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